1. Field of the Invention
The present invention relates to a method of manufacturing a lithium ion secondary battery, the lithium ion secondary battery including: a positive electrode that includes a positive electrode active material layer containing positive electrode active material particles; a negative electrode; and a nonaqueous electrolytic solution that contains a compound containing fluorine.
2. Description of Related Art
It is known that, in a lithium ion secondary battery (hereinafter, referred to simply as “battery”), the positive electrode potential is high; therefore, a nonaqueous solvent of a nonaqueous electrolytic solution is likely to be oxidized and decomposed on particle surfaces of positive electrode active material particles. In a case where the nonaqueous electrolytic solution contains a compound containing fluorine, hydrogen ions, which are produced by the oxidative decomposition of the nonaqueous solvent, may react with fluorine to produce hydrofluoric acid (HF). As a result, due to the action of the hydrofluoric acid, a metal element such as a transition metal is eluted from the positive electrode active material particles, and the battery capacity decreases. Therefore, this battery has a problem in that the battery capacity significantly decreases in a charging-discharging cycle test.
In order to solve the problem, a technique of adding particles of a metal phosphate such as lithium phosphate or a metal pyrophosphate to the positive electrode active material layer in advance is known. When metal phosphate particles are added to the positive electrode active material layer, the above-described hydrofluoric acid reacts with a metal phosphate during the initial charging of the battery, and a film containing fluorine and phosphorus is formed on particle surfaces of the positive electrode active material particles. This film prevents direct contact between the nonaqueous electrolytic solution and the positive electrode active material. Therefore, even when the positive electrode potential exceeds an oxidative decomposition potential of the nonaqueous solvent after the formation of the film, the oxidative decomposition of the nonaqueous solvent can be prevented. Accordingly, after the charging-discharging cycle test is performed on the battery, a decrease in battery capacity can be reduced. For example, Japanese Patent Application Publication No. 2014-103098 (JP 2014-103098 A) discloses a technique of adding particles of a metal phosphate such as lithium phosphate or sodium phosphate to a positive electrode mixture layer (positive electrode active material layer).
However, it was found that, when the charge current increases during the initial charging of a battery, the battery resistance tends to increase. The film containing fluorine and phosphorus is a resistor. However, it is presumed that, when the charge current is high during the formation of the film, the oxidative decomposition of a nonaqueous electrolytic solution excessively occurs, and the thickness of the film increases; therefore, the electric resistance increases.